For the past 20 years, the integrated circuit (IC) device density has doubled about every 18 months. When the gate length of integrated circuits is less than 0.18 .mu.m, the propagation time or delay time is dominated by interconnect delay instead of device gate delay. To address this problem, new materials with low dielectric constants are being developed. The aim of this development is to decrease time constant (RC delay), decrease power consumption, and decrease cross-talk in integrated circuits. There are two groups of low K dielectric materials. These include the traditional inorganic group exemplified by SiO.sub.2, and newer organic polymers, exemplified by poly(para-xylylene). Organic polymers are considered an improvement over inorganic low dielectric materials because the K of organic polymers can be as low as 2.0. However, most of the currently available organic polymers have serious problems. Specifically, they have insufficient thermal stability, and are difficult and expensive to manufacture in a vacuum system.
For IC features of 0.35 .mu.m, current production lines use materials consisting primarily of SiO.sub.2. The SiO.sub.2 products have dielectric constants ranging from 4.0 to 4.5. In addition, stable fluorinated SiO.sub.2 materials with a dielectric constant of 3.5 have been achieved. These SiO.sub.2 -containing materials are primarily obtained from plasma enhanced chemical vapor deposition (PECVD), photon assisted chemical vapor deposition (PACVD), and high density plasma chemical vapor deposition (HDPCVD) of various silanes, siloxanes and hydrocarbons.
I. Precursors and Polymers
During the past few years, several types of precursors have been used to manufacture polymers with low dielectric constants for use in manufacture of integrated circuits (IC). Transport Polymerization (TP) and Chemical Vapor Deposition (CVD) methods have been used to deposit such low dielectric materials. The starting materials, precursors and end products fall into three groups, based on their chemical compositions. The following examples of these types of precursors and products are taken from Proceedings of the Third International Dielectrics for Ultra Large Scale Integration Multilevel Interconnect Conference (DUMIC), Feb. 10-11 (1997).
A. Modification of SiO.sub.2 by Carbon (C) and Fluorine (F)
The first method described is the modification of SiO.sub.2 by adding carbon and/or fluorine atoms. McClatchie et al., Proc. 3d Int. DUMIC Conference, 34-40 (1997) used methyl silane (CH.sub.3 --SiH.sub.3) as a carbon source, and when reacted with SiH.sub.4 and the oxidant H.sub.2 O.sub.2 and deposited using a thermal CVD process, the dielectric constant (K) of the resulting polymer was 3.0. However, this K is too high to be suitable for the efficient miniaturization of integrated circuits.
Sugahara et al., Proc. 3d Int. DUMIC Conference, 19-25 (1997) deposited the aromatic precursor, C.sub.6 H.sub.5 --Si--(OCH.sub.3).sub.3 on SiO.sub.2 using a plasma enhanced (PE) CVD process that produced a thin film with a dielectric constant K of 3.1. The resulting polymer had only fair thermal stability (0.9% weight loss at 450.degree. C. in 30 minutes under nitrogen). However, the 30 min heating period used in the evaluation of thermal stability is shorter than the time needed to manufacture complex integrated circuits. Multiple deposition steps, annealing, and metalizing steps significantly increase the time during which a wafer is exposed to high temperatures. Thus, this dielectric material is unsuitable for manufacture of multilevel integrated circuits.
Shimogaki et al., Proc. 3d Int. DUMIC Conference, 189-196 (1997) modified SiO.sub.2 using CF.sub.4 and SiH.sub.4 with NO.sub.2 as oxidant in a PECVD process. The process resulted in a polymer with a dielectric constant of 2.6, which is lower than that of SiO.sub.2.
However, one would expect low thermal stability due to low bonding energy (BE) of sp.sup.3 C--F and sp.sup.3 C--Si bonds (BE=110 and 72 kcal/mol., respectively) in the film. The low thermal stability would result in films which could not withstand the long periods at high temperatures necessary for integrated circuit manufacture.
B. Amorphous-Carbon (.alpha.C)- and Fluorinated Amorphous Carbon (F-.alpha.C)-Containing Low Dielectric Materials
The second approach described involves the manufacture of .alpha.-carbon and .alpha.-fluorinated carbon films. Robles et al., Proc. 3d Int. DUMIC Conference, 26-33 (1997) used various combinations of carbon sources including methane, octafluorocyclobutane and acetylene with fluorine sources including C.sub.2 F.sub.6 and nitrogen trifluoride (NF.sub.3) to deposit thin films using a high density plasma (HDP) CVD process.
The fluorinated amorphous carbon products had dielectric constants as low as 2.2 but had very poor thermal stability. These materials shrank as much as 45% after annealing at 350.degree. C. for 30 minutes in nitrogen.
One theory which could account for the low thermal stability of the fluorinated amorphous carbon products is the presence of large numbers of sp.sup.3 C--F and sp.sup.3 C--sp.sup.3 C bonds in the polymers. These bonds have low bonding energy and therefore cannot withstand the long periods of high temperatures necessary for IC manufacture.
Several thermally stable polymers or polymer precursors are under study. These include polyimides (PIM), fluorinated polyimides (F-PIM), polyquinoxalines (PQXL), benzocyclobutenes (BCB), fluorinated polyphenylethers (F-PPE), and several types of silsesquisiloxanes. These polymers have dielectric constants ranging from 2.6 to 3.0. Solutions of these polymers or their precursors are used in spin coating processes to achieve gap filling and planarization over metal features. However, the dielectric constants of these polymers are too high for the future ICs with small feature sizes. In addition, all thermally stable polymers including PIM and PQXL have a persistent chain length (PCL or the loop length of a naturally curling up polymer chain) up to several hundred or thousands of .ANG.. Long PCL makes complete gap filling very difficult if not physically impossible.
C. Polymers Containing Aromatic Moieties
Recently, other types of low dielectric material, poly(para-xylylenes) (PPX) have been studied and evaluated for future IC fabrication. These PPX include Parylene-N.TM., Parylene-C.TM. & Parylene-D.TM. (trademarks of Special Coating System Inc.'s poly(para-xylylenes). Currently, all commercially available poly(para-xylylenes) are prepared from dimers. The currently available starting materials or dimers for manufacturing poly(para-xylylenes) are expensive (more than $500 to $700/kg). Unfortunately, these poly(para-xylylenes) have dielectric constants (K=2.7-3.5) and low thermal stability. The decomposition temperature, Td is less than 320.degree. C.-350.degree. C. in vacuum, and thus these materials are not suitable for IC fabrication requiring high temperature processing. The fluorinated poly(para-xylylenes) (F-PPX) or Parylene AF-4.TM. for example, has the structure of (--CF.sub.2 --C.sub.6 H.sub.4 --CF.sub.2 --).sub.n. It has a dielectric constant of 2.34 and is thermally stable (0.8%/hr. wt. loss at 450.degree. C. over three hours in nitrogen atmosphere).
II. Processes for Manufacturing Polymers
Currently, fluorinated poly(para-xylylenes) are polymerized from F-dimers by the method of Gorham, (J. Polymer Sci. A1(4):3027 (1966)) as depicted in Reaction 1 below: ##STR1##
In this reaction, Ar is --C.sub.6 H.sub.4 --. However, the precursor molecule and the F-dimer needed for the manufacture of Parylene AF-4.TM. is expensive and time-consuming to make because several chemical reaction steps are needed to make its fluorinated dimer.
F-dimers are manufactured according to the following series of chemical steps: ##STR2##
The overall yield for making F-dimers is low (estimated from 12% to 20% based on the weight of its starting material). In addition, the last step of the syntheses of the precursor, or the dimerization step (4a or 4b) can only be effectively carried out in very dilute solutions (from 2% to less than 10% weight/volume) resulting in low conversion efficiency. Further, the needed lead time and material cost for making F-containing dimers is very high. For instance, 10 g of the F-dimer can cost as much as $2,000/g. The lead time is 2-3 months for getting 1 kg of sample from current pilot plant production facilities.
Therefore, even though fluorinated poly(para-xylylenes) might be suitable as dielectric materials in "embedded" IC structures, it is very unlikely that the F-dimer will ever be produced in large enough quantity for cost-effective applications in future IC fabrication.
On the other hand, a readily available di-aldehyde starting material (Compound Ia) is reacted with sulfurtetrafluoride at elevated pressure of 1 MPa to 20 MPa and temperatures of 140.degree. C. to 200.degree. C. to yield the tetrafluorinated precursor (Compound IIIa) and sulfur dioxide (Reaction 2). The sulfur dioxide is then exhausted from the reaction chamber. Alternatively, the di-aldehyde can be reacted with diethylaminosulfur trifluoride (DAST) at 25.degree. C. at atmospheric pressure to make the Compound IIIa. ##STR3##
Y is a leaving group, and Ar is a phenylene moiety. Both Compound Ia and Compound IIIa have non-fluorinated phenylene moieties. The Compound IIIa in solution can be converted into a dibromo Compound IIIb (see below, Reaction 3) through a photo-reaction (Hasek et al., J. Am. Chem. Soc. 82:543 (1960). The dibromo Compound IIIb (1-5%) was used in combination with CF.sub.3 --C.sub.6 H.sub.4 --CF.sub.3 by You, et al., U.S. Pat. No. 5,268,202 to generate di-radicals (Compound IV) that were transported under low pressure to a deposition chamber to make thin films of fluorinated poly(para-xylylenes). ##STR4##
Additionally, poly(para-xylylene)-N (Parylene-N.TM. or PPX-N) was also prepared directly from pyrolysis of p-xylene. (Errede and Szarwe, Quarterly Rev. Chem. Soc. 12:301 (1958); Reaction 4). According to this publication, highly cross-linked PPX-N was obtained. ##STR5## III. Deposition of Polymer Films
The deposition of low dielectric materials onto wafer surfaces has been performed using spin on glass (SOG), but for newer devices which have features smaller than 0.25 .mu.m, SOG processes cannot fill the small gaps between features. Therefore, vapor deposition methods are preferred. Of these, transport polymerization (TP) and chemical vapor deposition (CVD) are most suitable.
In both TP and CVD, the precursor molecule is split (cracked) to yield a reactive radical intermediate which upon deposition onto the wafer can bind with other reactive intermediate molecules to form a polymer. The polymer thus forms a thin film of material with a low dielectric constant.
Chemical vapor deposition has been used to deposit thin films with low dielectric constant. Sharangpani and Singh, Proc. 3d Int. DUMIC Conference, 117-120 (1997) reported deposition of amorphous poly(tetrafluoroethylene) (PFTE; Teflon.TM., a registered trade name of DuPont, Inc.) by a direct liquid injection system. A solution of PFTE is sprayed on a wafer substrate, which is exposed to ultraviolet light or with light from tungsten halogen lamps. Unfortunately, PFTE has a low glass transition temperature (Tg) and cannot be used for IC fabrication requiring temperatures of greater than 400.degree. C.
Labelle et al., Proc. 3d Int. DUMIC Conference, 98-105 (1997) reported using pulsed radio frequency (RF) plasma enhanced CVD (PECVD) process for deposition of hexafluoropropylene oxide. However, as with poly(tetrafluoroethylene), the resulting polymers have low Tg values and cannot be used as dielectrics.
Kudo et al., Proc. 3d Int. DUMIC Conference, 85-92 (1997) reported using a PECVD process for deposition of hydrocarbons including C.sub.2 H.sub.2 /(C.sub.2 H.sub.2 +C.sub.4 F.sub.4).
Lang et al., Mat. Res. Soc. Symp. Proc. 381:45-50 (1995) reported thermal CVD process for deposition of poly(naphthalene) and poly(fluorinated Naphthalene). Although polymers made from these materials have low dielectric constants, the polymers are very rigid, being composed of adjoining naphthalene moieties. Thus, they are prone to shattering with subsequent processing such as Chemical Mechanical Polishing (CMP).
Selbrede and Zucker, Proc. 3d Int. DUMIC Conference, 121-124 (1997) reported using a thermal TP process for deposition of Parylene-N.TM.. The dielectric constant of the resulting polymer (K=2.65-2.70) also was not low enough. For future IC applications, the decomposition temperature (Td) of the thin film was also too low to withstand temperatures greater than 400.degree. C.
Wang et al., Proc. 3d Int. DUMIC Conference, 125-128 (1997) reported that annealing a deposited layer of poly(para-xylylene) increases the thermal stability, but even then, the loss of polymer was too great to be useful for future IC manufacturing.
Wary et al. (Semiconductor International, June 1996, pp: 211-216) used the fluorinated dimer, the cyclo-precursor (.varies., .varies., .varies.', .varies.', tetrafluoro-di-p-xylylene) and a thermal TP process for making polymers of the structural formula: {--CF.sub.2 --C.sub.6 H.sub.4 --CF.sub.2 --}.sub.n. Films made from Parylene AF-4.TM. have dielectric constant of 2.28 and have increased thermal stability compared to the hydrocarbon dielectric materials mentioned above. Under nitrogen atmosphere, a polymer made of Parylene AF-4.TM. lost only 0.8% of its weight over 3 hours at 450.degree. C.
All current commercial or laboratory deposition systems used for transport polymerization of dimers primarily consist of (1) a vaporizer for the solid dimers, (2) a pyrolyzer to crack the dimers and (3) a deposition chamber. The configuration for a commonly used commercial system is shown in the attached FIG. 1. FIG. 1 shows a general diagram of a prior art transport polymerization system 100 using solid dimers. A door 104 permits the placement of precursors into the vaporizer 108. The vaporized precursors are transported to the pyrolyzer 112, where the precursors are thermally cleaved into reactive intermediates. The intermediates are then transported via a pipe 116 to the chamber 120 and chuck 124, where the intermediates polymerize on the wafer surface. A valve 132 permits the chamber pressure to be lowered by a dry pump 136 keeps the pressure of the system low, and the cold trap and mechanical chiller 128 protects the pump from the unpolymerized molecules in the chamber.
In addition, You and his coworkers patented a so called "one chamber system" for transport polymerization of liquid monomers such as Dibromotetrafluor-p-xylene (DBX) and 1,4-bis-(trifluoromethyl) benzene (TFB) U.S. Pat. No.: 5,268,202). In their deposition system, shown in FIG. 2, both the pyrolyzer and the wafer are situated inside the same vacuum chamber. The system also utilizes a resistive heater to crack the DBX and TFB.
The reactor of You et al. comprises a vacuum chamber 10 containing a reactor 12 which contains a metal catalyst 16. The reactor is heated by a resistive heater 18 and a heat shield 22 surrounds the reactor. Precursors are fed into the reactor via a reactor supply tube 24 and are stored in a storage container 26. Flow of precursors from the storage container into the reactor is regulated by a control valve 28. The reactor has an outlet 29 through which dissociated precursors flow. A shutter 30 is used to protect the wafer 14 from being exposed to the high heat of the reactor, to keep metal catalyst ions inside the reactor, and to act as a diffusion plate. The wafer 14 is held on a cooling device 34 which keeps the temperature of the wafer below that of the reactor. An outlet port 44 is disposed on the bottom of the vacuum chamber and is connected to mechanical and diffusion pumps.
However, the resistive heater has very low heating rate and long temperature stabilization time. Thus, it is not suitable for future IC manufacturing equipment. Furthermore, all current pyrolyzers utilize metal parts which potentially leach out metal ions under high temperature (&gt;600 to 800.degree. C.). These metal ions result in metallic contamination of deposited thin films. Moreover, the precursor inlet port and outlet port 44 are on the same end of the chamber, namely at the end opposite the end where the wafer is held. Further, the wafer is protected by a heat shield which must be kept close to the heat source, and thus, is not ideally suited to act as a diffusion plate to ensure the even distribution of intermediates onto the wafer surface. Thus, deposition of precursors onto the wafer surface is not easily regulated and the thickness of dielectric films cannot be made constant over the entire wafer surface.
In contrast to a CVD process, transport polymerization (TP) (Lee, C. J., Transport Polymerization of Gaseous Intermediates and Polymer crystal Growth." J. Macromol. Sci.--Rev. Macromol. Chem. C16:79-127 (1977-1978), avoids several problems by cracking the precursor in one chamber and then transporting the intermediate molecules into a different deposition chamber. By doing this, the wafer can be kept cool, so that metal interconnect lines on the wafer are not disrupted, and multiple layers of interconnect films may be manufactured on the same wafer. Further, the conditions of cracking can be adjusted to maximize the cracking of the precursor, ensuring that very little or no precursor is transported to the deposition chamber. Moreover, the density of the transported intermediates may be kept low, to discourage re-dimerization of intermediates. Thus, the thin films of low dielectric material are more homogeneous and more highly polymerized than films deposited by CVD. These films have higher mechanical strength and can be processed with greater precision, leading to more reproducible deposition and more reproducible manufacturing of integrated circuits.
Among all currently available poly(para-xylylenes), F-PPX ((--CF.sub.2 --C.sub.6 H.sub.4 --CF.sub.2 --).sub.n or Parylene AF-4.TM.) has the lowest dielectric constant and best thermal stability. This is due to a lower polarity and higher bonding energy of C--F bond compared to those of C--H bond. So far, the F-PPX is considered to be the most promising "embedded" IMD for future 0.18 .mu.m ICs due to its low dielectric constant (K=2.34) and high thermal stability (0.8%/hr. wt. loss at 450.degree. C. up to 3 hours). However, to be useful as interlevel dielectric materials, a lower K (K&lt;2.3-2.5) polymer still needs to have better thermal stability, T.sub.d and thermal mechanical strength than those of the Parylene AF-4.TM.. Higher T.sub.d, glass transition temperature T.sub.g and Elastic Modulus are needed for re-flow or annealing of aluminum or copper. In addition, higher Tg and Elastic Modulus (E) are desirable for CMP to achieve global planarization. The development of new precursors and polymers requires new equipment for their dissociation and deposition, respectively. Certain of the precursors of the co-pending applications require carefully controlled conditions of precursor dissociation. Those precursors are described in the above identified co-pending applications. Such careful control over process conditions are impossible using conventional equipment. In this invention, new equipment for dissociation of precursors and deposition of polymers are provided to overcome the above mentioned problems.